Fixes in babeltrace core to support non-CTF traces

Since we will have an effort soon to cleanup the Babeltrace plugin
mechanism, this patch adds just the bits required in Babeltrace to not
crash if a non-ctf is opened.
That way, an input plugin can just implement the open_trace callback and
process its input trace.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>